Oracle - Undo questions

I have a ver big Insert using APPENd hint running on Database A and it is selecting data from Database B using Dblink from multiple Tables with lots of joins/subqueries and inserting several million rows (around 5-10 Mil). It failed few time with ORA-0155 error. 1. Does that mean issue with UNDO on Database A or B?2. Is there way to troubleshoot and get some sizing estimate for UNDO on Database A or Database B(Based on Point 1) that give some clear picture like quering v$undostat or so if the issue happened today and Undostat have anyway data for few days. Thanks for the time
Sanjay

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